This invention relates to optical scanners of the type in which a beam of light is repetitively scanned across a moving photosensitive recording element to record video information thereon. More particularly, this invention relates to improvements in apparatus for accurately positioning the light beam prior to each scanning movement thereof.
For many years now, optical scanners have been used to record video information on photosensitive recording elements. Such scanners typically employ a light-deflecting device (e.g. a rotating polygonal mirror, an oscillating galvanometer mirror, an acoustooptic cell, etc.) to repetitively scan a light beam across a photosensitive surface while the beam is intensity-modulated with video information. Each scanning movement of the beam produces upon the recording element a linear scan line which, in conventional notation, extends in the X (i.e. horizontal) direction. To space the scan lines and thereby effect two-dimensional imaging, it is common to advance the recording element in the Y (i.e. vertical) direction while the horizontal scan lines are produced. By advancing the recording element at a constant rate, a uniform scan line spacing can be achieved. This assumes, of course, that the deflection device is capable of scanning the beam in the same horizontal position, scan after scan after scan. For high quality imaging, it has been found that the scan line position must be controlled to within a few seconds of arc, from scan to scan.
The deflection device most commonly used in high speed optical scanners is the rotating polygonal mirror. To achieve the scanning accuracy mentioned above, it is, of course, necessary that this mirror be fabricated with an exceptional degree of accuracy. Not only must the individual reflective facets be smooth, flat and identical in size, but also the angular relationship between the plane of each individual facet and the axis of rotation of the mirror, must be virtually identical, from facet to facet. Small angular deviations can give rise to noticeable variations in scan line spacing which recur with each revolution of the mirror. As one would except, polygonal mirrors which are capable of repetitively scanning light beams with the aforementioned accuracy are very difficult and costly to manufacture.
To avoid the high costs associated with the manufacture of high quality polygonal mirrors, considerable effort has been expended heretofore in devising schemes for compensating for the angular defects inherent in low-cost mirrors. Such schemes accept the fact that most of the reflective facets of a low-cost polygonal mirror will produce a scan line which is somewhat displaced from a nominal position, and suggest apparatus for adjusting the system components to bring the scan line and the nominal position into coincidence. One such apparatus is disclosed in an article by Helmberger et al, entitled "Correction of Axial Deflection Errors in Rotating Mirror Systems," Optics and Laser Technology, December 1975. Such apparatus utilizes a preprogrammed acoustooptic cell to control the angle of incidence between a light beam and the facets of a rotating polygonal mirror. By controlling this angle of incidence, the cell controls the plane in which the reflected beams scans the recording element and, hence the line spacing. As each mirror facet is rotated into a position to scan the light beam, an error signal, proportional to the angular defects to such mirror facet (as determined by a precalibration procedure) is applied to the acoustooptic cell to adjust the angle at which the light beam strikes the facet. In this manner, the beams reflected by the mirror facets can be made to scan the same position, notwithstanding facet defects which would tend to cause the beam to scan above or below such position.
While the aforedescribed apparatus of Helmberger et al permits a relaxation of the manufacturing tolerances of the polygonal mirror, such apparatus tends to be relatively expensive, requiring factory calibration and a memory capability. Further, inasmuch as the Helmberger et al apparatus is an open loop system, the apparatus is not capable of compensating for dynamic changes in the scan line position, as may be occasioned, for instance, by changes in the ambient operating conditions or wear of the rotary mirror bearing.